Meghan Hodges
Air pollution is defined as the presence of any particle or gas in the air that is not part of its original composition. Air pollution can be classified in a variety of ways, and understanding the different types of air pollutants is crucial for effective regulation and mitigation strategies. Two broad categories of air pollution are primary and secondary pollutants, and common and hazardous air pollutants. Primary pollutants are emitted directly into the atmosphere by the original source, while secondary pollutants are formed due to reactions between primary pollutants and other elements in the atmosphere. Common air pollutants, as defined by the US EPA, include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are regulated by the National Ambient Air Quality Standards and can cause harm to human health, the environment, and property. Hazardous air pollutants, on the other hand, are known or suspected to cause serious health and environmental effects, including cancer and reproductive issues. These pollutants are regulated through amendments to the Clean Air Act.
| Characteristics | Values |
|---|---|
| Physical composition | Solid matter or gaseous particles |
| Sources | Natural (e.g. wildfires, pollen, volcanic eruptions) or man-made (e.g. vehicle emissions, industrial processes, construction) |
| Health impacts | Serious health effects including cancer, reproductive issues, birth defects, headaches, fatigue, impaired vision, and death |
| Regulation | Regulated by organisations such as the USEPA and U.S. Clean Air Act |
| Common Air Pollutants | Particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead |
| Hazardous Air Pollutants (HAPs) | Benzene, dioxins, asbestos, toluene, cadmium, mercury, chromium, lead compounds |
| Primary Pollutants | Carbon monoxide, sulfur dioxide, volatile organic compounds (VOCs), nitrous oxides (NOx) |
| Secondary Pollutants | Ground-level ozone, photochemical smog |
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What You'll Learn
Primary and secondary pollutants
Air pollutants can be classified in various ways, such as their physical composition, their sources, their health impacts, and how they are regulated. One of the most common ways is to divide them into two categories: primary and secondary pollutants.
Primary Pollutants
Primary air pollutants are formed and emitted directly from specific sources. Examples of primary pollutants include particulates, carbon monoxide, nitrogen oxide, and sulfur oxide. These pollutants are often the result of fuel combustion in motor vehicles, power plants, and other industrial processes.
Primary pollutants can also be emitted from stationary sources, such as industrial facilities, power plants, and hazardous waste incinerators. These sources emit significant amounts of toxic pollutants into the air, especially in heavily populated areas. Natural sources of primary pollutants include dust storms, wildfires, and volcanic eruptions, which are becoming more frequent due to climate change, further impacting air quality.
Secondary Pollutants
Secondary air pollutants, on the other hand, are formed in the lower atmosphere by chemical reactions. Ground-level ozone, for instance, is a major secondary pollutant that forms when volatile organic compounds (VOCs) and nitrous oxides (NOx) interact with sunlight and heat. This process can lead to high ozone levels even in rural areas, as these primary pollutants can be carried long distances by wind.
Other secondary pollutants include haze, which is a type of secondary organic aerosol, and fine particulate matter (PM2.5). PM2.5 is composed of particles with an aerodynamic diameter of 2.5 microns or less, resulting from combustion activities and certain industrial processes.
Impact of Primary and Secondary Pollutants
Both primary and secondary pollutants contribute to the overall decline in air quality. For example, photochemical smog, which is prevalent in highly industrialised cities, is formed from interactions between primary pollutants like nitrogen oxides and secondary pollutants like ozone. This smog can have painful effects on the eyes and cause respiratory issues.
Additionally, secondary pollutants are harder to control because their formation processes are not yet fully understood, and they occur naturally in the environment. The complex interactions between different pollutants, wind patterns, and climatic factors further complicate the understanding and management of air quality.
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Common and hazardous pollutants
Air pollutants can be classified in several ways, including their physical composition, their sources, their health impacts, and how they are regulated. One such classification categorises pollutants as either common or hazardous.
Common Pollutants
Common air pollutants are those that are regulated by the National Ambient Air Quality Standards. They include:
- Particulate matter: Made up of tiny pieces of solids or liquids in the air, including smoke, dust, and other particles. These can irritate the eyes, nose, and throat, and smaller particles can get into the deep parts of the lungs or even the bloodstream.
- Ground-level ozone: A secondary pollutant formed when volatile organic compounds (VOCs) and nitrous oxides (NOx) react with sunlight and heat. Ground-level ozone is harmful to both human and environmental health.
- Carbon monoxide
- Sulfur dioxide
- Nitrogen dioxide
- Lead
Hazardous Pollutants
Hazardous air pollutants, also known as toxic air pollutants, are those known or suspected to cause serious health effects, including cancer, birth defects, or other adverse environmental impacts. Examples of hazardous air pollutants include:
- Benzene
- Dioxins
- Asbestos
- Toluene
- Metals such as cadmium, mercury, chromium, and lead compounds
- Hydrogen chloride
Hazardous air pollutants come from a variety of sources, including industrial emissions, vehicle emissions, and indoor sources such as tobacco smoke and cleaning products.
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Particle and ground-level ozone pollution
Air pollution is a complex and dynamic issue that can be classified in various ways, such as the physical composition of the pollutants, their sources, health impacts, and regulatory considerations. One of the key classifications distinguishes between common air pollutants and hazardous air pollutants. Particle pollution and ground-level ozone pollution are two critical components within these categories.
Particle pollution, also known as particulate matter (PM), is a complex mixture of small particles, water vapour, and gases. These particles can include lead, dust, dirt, and sand, which, when combined, contribute to air pollution. The particles that make up PM vary in size, ranging from ultrafine to coarse. PM also differs in composition, with certain types, such as black carbon, being particularly toxic. Particle pollution is recognised as one of the six "criteria pollutants" by the USEPA due to its widespread impact on human health and the environment.
Ground-level ozone, on the other hand, is a secondary pollutant formed through chemical reactions. It is the product of interactions between primary pollutants, volatile organic compounds (VOCs), and nitrous oxides (NOx) in the presence of sunlight and heat. Ground-level ozone is often referred to as the "bad" ozone as it is harmful to both human health and the environment. It is a significant component of smog and acts as a dense barrier, trapping heat and reducing visibility.
Ground-level ozone is not directly emitted into the air but is created through chemical reactions involving pollutants from cars, power plants, industrial boilers, refineries, and chemical plants. It can also be formed by everyday items like paints, cleaners, solvents, and lawn equipment. As a result, ground-level ozone concentrations tend to be highest near urban centres. However, it can be transported long distances by wind, affecting even rural areas.
The health impacts of ground-level ozone pollution are significant. It can cause respiratory issues such as coughing, shortness of breath, and poor breathing, particularly in children, the elderly, and people with lung diseases. Prolonged exposure can lead to more severe conditions, including damaged airways, chronic bronchitis, asthma, and emphysema. Even after symptoms subside, ozone can continue to inflict damage on lung tissue.
To address ground-level ozone pollution, the EPA has established regulations and standards, including the National Ambient Air Quality Standards (NAAQS). These standards help states implement measures to reduce ozone levels and improve air quality. The EPA designates areas as attainment or nonattainment based on whether they meet the national standards. States with nonattainment areas must develop state implementation plans (SIPs) to outline the steps they will take to improve air quality.
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Physical composition: solid, liquid, or gas
Air pollution is caused by solid and liquid particles, as well as certain gases that are suspended in the air. These particles and gases can come from car and truck exhaust, factories, dust, pollen, mould spores, volcanoes, and wildfires. The solid and liquid particles suspended in the air are called aerosols.
Aerosols can enter the atmosphere when fossil fuels such as coal and petroleum, as well as wood, are burned. They can also be formed through chemical reactions in the air. Many aerosols are harmful to both human health and the environment.
Particulate matter (PM), or particle pollution, includes all airborne substances that are not gases. It is a mix of microscopic solid particles or droplets suspended in a gas. These particles consist of a large variety of materials and chemical compounds, including toxic substances, and vary in size. Ultrafine particles are 0.1 micrometres (μm) or smaller, while coarse PM (PM10) is 10 micrometres or smaller. Smaller particles pose a greater risk to health as they can reach the bloodstream.
The main natural sources of PM are sea spray, wildfires, volcanoes, and dust storms. The main human sources are the burning of biomass and fossil fuels, road emissions, and dust resuspension. Human-made PM is usually finer than natural PM. A 1993 Harvard study established a definitive link between fine particulate pollution and higher death rates in urban areas.
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Natural and manmade sources
Air pollution can be classified in several ways, including by its physical composition, its sources, its health impacts, and how it is regulated. One way to classify sources of air pollution is by categorizing them as natural or man-made.
Natural Sources
Natural sources of air pollution include volcanic eruptions, which release sulfur dioxide, ash, and other pollutants into the atmosphere, contributing to regional and even global air quality issues. Wildfires, another natural source, release large amounts of particulate matter, carbon monoxide, and volatile organic compounds (VOCs), affecting both air quality and human health. Wildfires can also reduce visibility and have been linked to an increased risk of dementia. Dust storms, driven by natural weather patterns, can transport vast amounts of dust and particulates over long distances, impacting air quality in various regions. Natural fog, pollen grains, and bacteria are other examples of natural sources of air pollution. Pollen grains, in particular, can cause bronchitis, asthma, and dermatitis in humans.
Man-made Sources
Man-made sources of air pollution encompass industrial activities, with factories emitting pollutants such as sulfur dioxide, nitrogen oxides, and particulate matter into the air. Transportation, particularly vehicles with internal combustion engines, releases significant amounts of carbon monoxide, nitrogen oxides, and VOCs, contributing to urban air quality issues. Mobile sources, such as cars, buses, planes, trucks, and trains, account for more than half of all air pollution in the United States, according to the Environmental Protection Agency. Stationary sources, like power plants, oil refineries, and industrial facilities, emit large amounts of pollution from a single location and are also known as point sources. Area sources, such as agricultural areas, cities, and wood-burning fireplaces, are made up of smaller pollution sources that can collectively have a significant impact.
Some specific examples of man-made sources of air pollution include ports, which often rely on fossil fuels and diesel to power ships, trucks, and other vehicles. Ships alone account for a large percentage of diesel particulate matter, NOx, and SOx emissions associated with ports. Additionally, industrial processes such as oil and gas development can lead to elevated ozone concentrations. Coal-powered plants have been linked to various health risks, including an increased mortality risk, cancer, cardiovascular disease, respiratory diseases, and more.
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Frequently asked questions
Air pollution can be classified as either primary or secondary. Primary air pollution is emitted directly into the atmosphere by the original source, whereas secondary air pollution is formed due to reactions between primary pollutants and other elements in the atmosphere, such as ozone.
Another way to classify air pollution is by its physical composition: air pollutants can be composed of solid matter or gaseous particles.
Primary air pollutants include volatile organic compounds (VOCs) and nitrous oxides (NOx), which can be transported long distances by wind. Other examples include carbon monoxide, which is present in car exhaust and smoke, and sulfur dioxide, which is produced when coal and fuel oils are burned.
Ground-level ozone is a major secondary air pollutant that forms due to interactions between primary pollutants such as VOCs and NOx with sunlight and heat. Photochemical smog is another example of secondary air pollution, formed primarily from VOCs and NOx, which accounts for most of the smog we see today.
